Conventionally, as a brushless motor driving apparatus of a sensorless system, there has been known a configuration that, for a duration of a rated operation, compares a back electromotive force generated in a non-drive conditioned motor coil phase with a neutral voltage of star-connected three-phase motor coils, waits (a zero-cross detection) for a time that a polarity determined by the comparison is inverted, and with the polarity-inverted time as a rotor position detection signal, and further after an addition of an appropriately set delay time, performs a transition to a next driving condition (See Japanese Patent Application Publication Laid-open No. HEI 9-47078, for instance). This brushless motor driving apparatus is operative, at a starting time of the motor, to start the motor by switching a driving phase forcibly with no rotor position detected by a synchronizing signal obtained from a driving apparatus-side oscillator in a period from a start to a low speed region.
However, the brushless motor driving apparatus according to the related art of the above configuration requires that sensing be performed continuously until a zero-cross signal indicating an inversion of the polarity determined by the comparison between a detected value of the back electromotive force and the neutral voltage is obtained, so that there is no denying a weakness to an external noise in a period before the zero-cross signal is obtained. Further, the above period necessitates a suppression of noise generation and hardly changes an output driving condition, so that a performance, such as an efficient change of a driving duty with a load amount, also is hardly increased.
Further, no generation of the back electromotive force at the starting time leads to a need for a dedicated starting circuit, and, in addition to this circuit, back electromotive force detection amplifiers equivalent in number to phases and some function circuits, such as an offset correction circuit for the above amplifiers, a noise removal circuit and a driving phase correction circuit are also required. Accordingly, a circuit configuration is easily complicated as a whole of the apparatus. This means that a power saving, a cost reduction and a downsizing of the brushless motor driving apparatus are hardly achieved.
Furthermore, there is also the need for a connection to extract the neutral voltage, so that in the case of the three-phase motor, for instance, the number of connections to connect the coils to the driving apparatus basically comes to four. When the driving apparatus is integrated into a motor apparatus, the smaller number of connections increases a degree of freedom of a circuit arrangement. In the brushless motor driving apparatus according to the related art, an attempt to carry out three connections, when being made, raises problems such as a degraded driving efficiency and a degraded operation stability caused by an addition of the correction circuit for correcting a zero-cross detection error or an acceptance of the detection error as it is. Thus, a use of the four connections is supposed to be indispensable, resulting in a restriction on the degree of freedom of the circuit arrangement when the driving apparatus is integrated into the motor apparatus.
The present invention has been made in view of the above problems and is intended to provide a motor driving apparatus, which may perform stable driving for revolutions with a relatively simple circuit configuration without being affected by an external noise, enables a low power consumption, a downsizing and a cost reduction to be achieved, and, besides, ensures a high degree of freedom of a circuit arrangement, a driving method thereof and a mobile terminal mounted with the driving apparatus.